David Learmount/London Andrew Doyle/Munich Julian Moxon/Paris

The Aerospatiale/ British Aerospace Concorde crashed on 25 July after a 4.5kg (9.9lb) lump of tyre carcass penetrated a port wing fuel tank. The damage led to an underwing fire, and the Nos 1 and 2 engines lost power as a result of debris ingestion. It has also emerged that the crew shut down the No 2 engine having received a fire warning after rotation. Originally it was thought that the engine had run down.

The joint accident investigation teams recommended that Concorde's certificates of airworthiness (C of A) be suspended on 16 August.

On the orders of the French Transport Minister Jean-Claude Gayssot, Air France had grounded its Concorde fleet after the accident, whereas British Airways (BA) resumed operations the next day with UK Civil Aviation Authority approval. The decision to pull the C of A resulted in BA calling a Concorde back from the runway at London Heathrow as it prepared to take-off. The two airlines have 12 operable aircraft between them.

Industry sources say that the certification tests of the airframe damage effects of tyrebursts had not anticipated that such a heavy single piece of tyre material would be produced by failure of the tyre, but since these tests, the tyre design has been changed, making it less likely that tread would be shed and more likely that tyre carcass material would be projected. Test evidence had originally shown that a 1kg piece of tyre was more likely to penetrate than a larger segment.

Following the suspension of Concorde's C of A, the UK airline was quick to confirm that it will "urgently be seeking meetings with Concorde's manufacturers and the airworthiness authorities to... enable the aircraft to resume operations as soon as possible". Its Concorde flight technical and training manager Mike Bannister says: "We are extraordinarily keen to get the aircraft flying again as soon as possible consistent with meeting the safety requirements."

The CAA, meanwhile, says that it will "together with the DGAC (the French aviation authority) require the joint manufacturers BAE Systems, Airbus UK, and EADS France (formerly Aerospatiale) to recommend an action plan including appropriate measures which will ensure a satisfactory level of safety of Concorde as far as risks associated with tyre bursts are concerned". In France, which is responsible for Concorde's airframe design, the government and EADS France are meeting to draw up plans for future design changes to enable the aircraft to regain its C of A.

The bursting of the front inboard tyre of the port mainwheel bogie, "very probably because it ran over a piece of metal", has been revealed as "the primary cause of the accident", according to the accident investigators' interim report. Rubber from the tyre is known to have penetrated the wing underside and holed No5 fuel tank, says the UK's chief inspector of air accidents Ken Smart. This produced a vapour plume which ignited, but the source of ignition is not yet certain. One theory under investigation is that vortices in the area of the wheelbay could have created an ignitable fuel/air mixture, and if electrical cables in the bay or on the gear leg were damaged, that could have provided a spark to ignite the mixture. This has not been established, however, and neither has the origin of the metal on the runway.

Also confirmed by Smart in the official interim announcement is that debris from the tyre entered both engines. They have been removed for inspection to a hangar at Paris Le Bourget airport, but are expected eventually to go to lead manufacturer Rolls-Royce at Derby for detailed examination. No 2 engine shows evidence of soft material impacts, and the engine had stalled momentarily before it was shut down by the crew say French sources. No 1 engine has evidence of metal impacts, and this had run down by the time the aircraft crashed after taking off from nearby Paris Charles de Gaulle.

According to the French sources, preliminary information from the aircraft's flight data recorder suggests that for unexplained reasons during the take-off the pilots initiated rotation slightly before the target rotation speed (Vr) had been reached, though this has yet to be confirmed by investigators.

Smart says that the tyreburst was different from others in Concorde's history (Flight International 8-14 August, P4) because collateral airframe and engine damage has previously been caused by metal wheel fragments or by fractured wheel water-deflectors, and the aircraft has survived the damage. If a tyreburst alone causes Concorde to become unflyable, then the aircraft fails to meet the conditions of its airworthiness certificate.

With the aircraft grounded the search has started for a possible fix to get Concorde back into service. BA says, that it is "premature for us to be commenting" on the nature of modifications to protect against tyreburst damage. Air France is declining to comment.

On 17 August, the airline met the Air Accident Investigation Branch (AAIB) to determine what might be needed to make the aircraft safe. Officials were reluctant to predict how long it might take to develop acceptable modifications.

Meanwhile, despite BA's and French Government professed keenness to encourage the manufacturers to propose modifications, the airline, the airframe manufacturer and the tyre makers Goodyear (Air France) and Dunlop (BA) are unwilling to discuss what modifications might be practical. It is clear, however, that the practical fixes under consideration all involve tyre and gear design. Any strengthening of the underwing area, say industry sources, would be difficult, expensive, and the recertification procedures would be lengthy.

One avenue could be to explore the feasibility of using radial tyres which may be less prone to causing secondary damage in the event of a burst, say industry sources.

Meanwhile lawyers acting for the families of the 96 German passengers killed in the accident say they plan to sue Air France for DM600 million ($270 million) in compensation. A total of 113 people died in the crash.

Source: Flight International